This invention relates to power combiners, and more particularly to a power combiner for microwave amplifiers, either tube or solid state types, each of whose output is applied as an input to one of a plurality of inputs of the combiner. In particular, the combiner may be advantageously used to combine the power of low-power, broadband travelling wavetubes (TWTs). The combiner provides a single output power substantially equal to the sum of powers provided by the input amplifiers.
There presently exists a need to provide a source of RF energy over a wide frequency band, e.g., 2.0-20 GHz, at power levels substantially an order of magnitude greater (hundreds of watts continuous) than is capable of being provided by currently available sources. There is also a need to have a source of RF power over this frequency range which does not suffer total loss of power output in the event that the tube providing the power fails. Thus, even if a tube capable of providing the desired power level over the frequency band were available, a source of power such as provided by this invention which results in only a reduction in power in the event of a tube failure is preferrable to total loss of RF power.
A divider/combiner amplifier circuit having internally mounted semiconductor amplifiers is disclosed in U.S. Pat. No. 4,424,496. In this patent, the input signal is divided and applied to each of a plurality of solid state amplifying elements mounted in a plurality of isolated channels which are combined to provide a single output. Failure of one or more of the amplifying elements produces a gradual dimunition of output power. The internally mounted amplifiers of the amplifier circuit of the referenced patent limits the total power output and frequency band of the combiner to a multiple of the power capability of each of the semiconductor amplifiers contained within the divider/combiner. Since these amplifiers are generally of low power output, the total power from the divider/combiner is more limited than is desired in many applications. There may also be a limitation with respect to the available bandwidth obtainable from each of the semiconductor amplifiers. A further possible limitation of the divider/combiner amplifier circuit of the referenced patent is that the divider portion of the amplifier circuit reduces the input power from a single source to each of the semiconductor amplifiers. There is no provision in the amplifier of the referenced patent for providing input power to a passive combiner circuit from a plurality of external amplifiers.
High CW powers (500 W to 1 kW) over multi-octave frequency bands up to 20 GHz are desired in several microwave applications. Normally a high-power TWT is used, but only partially satisfies the power-bandwidth requirements. Also a single tube high-power TWT has limitations in terms of the life, reliability, efficiency, etc. An alternate approach, as provided by this invention, is to power combine mini-TWTs. Since these tubes are highly reliable, efficient, and perform well over multi-octave bands, the problem is transferred to the power combiner which should have bandwidth and high-average power handling capabilities among other features.
The technique of power combining several devices to yield higher power is commonly used with solid state devices, such as GaAs FETs, GaAs Impatts, and bipolar transistors. For instance, GaAs Impatts have been combined in a TM.sub.020 cavity to provide peak powers up to 1 kW at X-Band with 1% bandwidth. GaAs FET amplifiers are frequently combined using different versions of the radial combiner. Wilkinson, modified Wilkinson, and travelling wave combiner are other types of combiners normally used depending upon power and bandwidth requirements.
For applications which require high CW power handling (hundreds of watts continuous) over a multi-octave bandwidth the foregoing power combiners are inadequate. Each of the TWTs desired to be combined have outputs in the range of 50-250W CW, and it is essential that a high degree of isolation be maintained between the combiner input ports not only in the desired balanced mode of operation, but also when some of the TWTs have failed.